Portion of a watch casing



FIG. 1 is a perspective view of a portion of a watch casing showing my new design;

FIG. 2 is a top plan view thereof;

FIG. 3 is an end elevational view thereof; and,

FIG. 4 is a vertical cross-sectional view thereof.

The broken line showings are environmental only and form no part of the claimed design. 

The ornamental design for a portion of a watch casing, as shown and described. 